Stationary natural gas engines are currently used for a variety of industrial applications, such as for electrical power generation and for oil-field pumping applications. These large-bore engines, up to 20 cylinders and up to 20 MW in capacity, are typically operated around the clock. While overall efficiency and reliability are of primary importance, NOx emissions are becoming a major concern with impending stricter EPA and state emissions regulations and laws. To overcome this problem, some manufacturers have resorted to lean burn combustion or exhaust gas recirculation (EGR). Lean burn natural gas engines operate with a higher than average air-fuel ratio of 22:1 rather that 17:1 which is normal. The advantage is that lean burn engines operate at a lower combustion temperature which is a major parameter for reducing NOx compared to stoichiometric or rich burn engines. Lean burn operation also decreases specific fuel consumption and pollutant emissions, including CO emissions. However, the drawbacks include the loss of specific power and misfire at very lean operating conditions which result in high hydrocarbon (HC) emissions. NOx reduction using EGR is also not usually a desirable solution because it degrades engine reliability and increases maintenance.
Nitrogen injection into intake air for NOx reduction in diesel engines is generally known in the industry and has been the subject of research efforts (see, for example, U.S. Pat. No. 5,649,517, U.S. Pat. No. 6,055,808, WO 00031386A1, and WO 09967508A1). In a diesel engine, liquid fuel is sprayed into the combustion chamber after the piston compresses intake air to very high pressures and temperatures. When the fuel is injected into the combustion chamber at a desired injection timing, the fuel vaporizes and a flame front develops on the outer periphery of the spray where the local equivalence ratio close to 1.0 is established. However this diffusion limited flame front moves to the remainder of the cylinder rather quickly as the ensuing fuel from the injector vaporizes and mixes with the in-cylinder air charge.
In a spark ignited (SI) natural gas engine, a spark is generated after the incoming natural gas-air homogenous mixture is compressed significantly. A flame front develops around the spark and progressively moves outward burning the remainder of the mixture. The speed of the flame front is determined by mixture properties, such as, pressure, temperature and excess air ratio (λ). Consequently, there are important differences between diesel engines and natural gas engines which require a different approach than in conventional diesel engine practices.
As mentioned above, current industry practice for natural gas, SI low-NOx engines is to operate the engine with high excess air, or relatively lean. This reduces the NOx, but at the expense of lowered power production and increased HC emissions. This effect is due to the quenching that high excess air provides in the homogenous charged engine. In contrast to what has been previously proposed, introducing additional diluent (such as N2) into a low-NOx, lean burn natural gas engine aggravates the quenching that is already occurring with the result that unacceptable HC emissions, fuel efficiency, and potentially, misfire may occur.
Similarly, diesel engines operate with high excess air. Unlike the homogenously charged natural gas engine, the area in which the combustion reactions occur is relatively small as discussed above, allowing additional diluent to be introduced with significant NOx reduction effect, but at the price of fuel efficiency. Also, additional diluent (N2, for example) must be introduced in relatively large quantities given the high degree of excess air in the diesel engine.
Tests confirm the above reasoning, with a small 2% nitrogen enrichment to the intake air of a stoic-burn natural gas engine lowered NOx production by approximately 70%. Similarly, in the case of a diesel engine 2% enrichment lowered NOx by 60%; however, the associated fuel penalty was significant, as high as 5%.
Some of the additional combustion concepts currently studied are Homogeneous Charge Compression Ignition (HCCI), pilot ignited natural gas engines, and the previously described spark-ignited natural gas engines. Each group has its advantages and limitations. HCCI engines are limited by poor ignition and combustion control at high loads while promising relatively low emissions. Pilot ignited natural gas engines utilize a pilot fuel, usually diesel, to initiate combustion. These engines take advantage of the high compression ratios of compression ignition (CI) engines and hence attain fairly higher fuel conversion efficiencies. However, these engines need a major hardware change (and thus large capital investment) for their functionality, and thus these modalities suffer from substantial disadvantages as well.